1. Field of the Invention
This invention relates to radiation curable inkjet ink sets and their use in inkjet printing methods.
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
When jetting the inkjet ink onto an ink-receiver, the ink typically includes a liquid vehicle and one or more solids, such as dyes or pigments and polymers. Ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        solvent-based, the drying primarily involving evaporation;        oil-based, the drying involving absorption and penetration;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification; and        UV-curable, in which drying is replaced by polymerization.        
It should be clear that the first three types of ink compositions are more suitable for an absorbing ink-receiver, whereas hot melt inks and UV-curable inks can also be printed on non-absorbing ink-receivers. Due to thermal requirements posed by hot melt inks on the substrates, especially radiation curable inks have gained the interest of the industry in inkjet printing applications.
The behaviour and interaction of a UV-curable ink on a substantially non-absorbing ink-receiver was found to be quite complicated compared to water-based inks on absorbent ink-receivers. In particular, a good and controlled spreading of the ink on the ink-receiver proved to be problematic and adhesion problems were observed on using different types of non-absorbing ink-receivers.
One way to approach these problems is to develop and use different ink sets for different types of substrates, but this is a not a preferred solution since changing inks in the printer and print head is very time consuming and not really a viable solution for an industrial printing environment.
The adhesion may be influenced by using different polymerizable compounds, surfactants, binders and/or organic solvents. U.S. Pat. No. 6,814,791 (DOMINO PRINTING SCIENCES) discloses inkjet printing methods wherein the ink composition including methyl acetate as a solvent is printed upon substrates of propylene and ethylene. The use of a well-chosen solvent usually results in partial swelling or dissolution of the substrate surface which leads to better adhesion, but can also cause problems of blocked nozzles in the printhead due to evaporation of solvent. Other solvents, such as 2-butoxyethyl acetate, are disclosed by WO 2005/047405 (VUTEK).
It is known that the adhesion of radiation curable inks can also be promoted on polyvinyl chloride substrates when one or more monomers are used that are suitable for the swelling of the PVC substrate and which are selected from the group consisting of tetrahydrofurfuryl acrylate, 1,6-hexanediol diacrylate and N-vinyl caprolactam. However, adhesion on polycarbonate substrates is promoted when one or more monomers are used that are suitable for the swelling of the polycarbonate substrate and which are selected from the group consisting of propoxylated neopentyl glycol diacrylate, 2-phenoxylethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate and polyethyleneglycol diacrylate. As a consequence one has to make the “best possible” mixture of monomers suitable for both the swelling of polyvinyl chloride substrates and polycarbonate substrates. Often such a compromise, whereby acceptable adhesion is obtained on several ink-receivers by making a complex mixture of ingredients, has a negative effect on the dispersion stability of a pigmented inkjet ink.
Adhesion problems have also been associated with shrinkage of an ink-layer after radiation curing. In this aspect, cationic inks have been regarded to be superior in comparison to free radical polymerizable inks. EP 1705229 A (FUJI) discloses cationically polymerizable inkjet inks exhibiting good adhesion and storage stability.
Specific monomers for cationic inks exhibiting improved adhesion are disclosed by US 2004166253 (KONICA MINOLTA) wherein the curable ink-jet ink includes an epoxy compound containing an alicyclic epoxy group and an epoxyfied fatty acid ester group.
U.S. Pat. No. 6,310,115 (AGFA) discloses radiation curable inkjet ink compositions including radiation curable monomers containing vinylether and acrylate functions, which can be cured both by cationic polymerization and free radical polymerization.
Instead of adapting the inkjet inks, it has become the general approach to modify the surface chemistry of the ink-receiver either by a pre-treatment such as plasma or corona treatment or by applying a suitable surface layer, a so-called primer.
Corona discharge treatment and plasma treatment increase the cost, complexity and maintenance of the equipment used to process the substrates. Substrates may contain significant impurities or irregularities that may interfere with the treatment of the substrate, and hence not result to the uniform spreading and adhesion of ink.
The other possibility is the application of a primer prior to jetting the inkjet inks. Generally, the surface layer is coated and dried or cured before jetting the inkjet ink as, for example, in the inkjet printing process in EP 1671805 A (AGFA) and US 2003021961 (3M), but it can also remain a wet, un-cured surface layer as in WO 00/30856 (XAAR).
A single composition of a surface layer suitable for all the different substrates is however not available. WO 2006/111707 (SUN CHEMICAL) discloses a process of ink jet printing in which:    i) a primer is applied to a substrate material;    ii) ink is ink jet printed onto the primed substrate;    iii) a characteristic relating to print quality is evaluated;    iv) the composition of the primer is adjusted in dependence on the evaluated characteristic relating to print quality; and    v) the adjusted primer composition is applied to the substrate material and ink is ink jet printed onto the primed substrate material to give a printed product. Although it might be possible to solve all adhesion problems with this approach, the method of testing and adjusting the surface layer remains a time-consuming method and not really a viable solution for an industrial printing environment.
US 2003/199655 (NIPPON SHOKUBAI) discloses reactive diluent and curable resin compositions including vinylether acrylates.
WO 2007/048819 (HUNTSMAN) discloses low viscosity photocurable compositions for rapid prototyping techniques containing (i) a cationically curable component (ii) a free radically active component (iii) an antimony-free cationic photoinitiator and (v) a free radical photoinitiator.
Therefore, a need continues to exist for radiation curable inkjet inks that adhere well to multiple substrates. A simple and fast method for improving the adhesion is desirable.